Working the CCNA exams with GNS3 software is a great way. But, you need Cisco IOS images for GNS3.
You can use many Cisco IOS images on GNS3, but most of them may be problematic.
I would recommend reading the related article to download Cisco IOS images running in GNS3.
Hey there @TolgaBagci would it be possible to get some links updated for those firmwares? I'd really love to download them to use with GNS3.